Role of the Translationally Controlled Tumor Protein

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Role of the translationally controlled tumor proteinin DNA damage sensing and repairJie Zhanga,b,c, Sonia M. de Toledoa, Badri N. Pandey a, Guozheng Guob,c, Debkumar Paind, Hong Lie,and Edouard I. Azzama,1

Departments of aRadiology, dPharmacology and Physiology, and eBiochemistry and Molecular Biology, New Jersey Medical School Cancer Center, University of

Medicine and Dentistry of New Jersey, Newark, NJ 07103; andb

Department of Radiation Medicine andc

Ministry of Education Key Laboratory of HazardAssessment and Control in Special Operational Environment, School of Public Health, Fourth Military Medical University, Xi an Shaanxi 710032, PeoplesRepublic of China

Edited by James E. Cleaver, University of California, San Francisco, CA, and approved February 27, 2012 (received for review April 19, 2011)

The translationally controlled tumor protein (TCTP) is essential for

survival by mechanisms that as yet are incompletely defined.Here we describe an important role of TCTP in response to DNA

damage. Upon exposure of normal human cells to low-dose rays,

the TCTP protein level was greatly increased, with a significant

enrichment in nuclei. TCTP up-regulation occurred in a manner

dependent on ataxia-telangiectasia mutated (ATM) kinase and

the DNA-dependent protein kinase and was associated with pro-tective effects against DNA damage. In chromatin of irradiated

cells, coimmunoprecipitation experiments showed that TCTP forms

a complex with ATM and H2A.X, in agreement with its distinctlocalization with the foci of the DNA damage-marker proteins

H2A.X, 53BP1, and P-ATM. In cells lacking TCTP, repair of chromo-somal damage induced by rays was compromised significantly.

TCTP also was shown to interact with p53 and the DNA-binding

subunits, Ku70 and Ku80, of DNA-dependent protein kinase. TCTP

knockdown led to decreased levels of Ku70 and Ku80 in nuclei of

irradiated cells and attenuated their DNA-binding activity. It alsoattenuated the radiation-induced G1 delay but prolonged the G2delay. TCTP therefore may play a critical role in maintaining geno-

mic integrity in response to DNA-damaging agents.

low dose ionizing radiation | adaptive responses | DNA repair |cell cycle checkpoints | genomic stability

Ionizing radiation is used extensively to investigate the molec-ular events involved in the sensing and repair of DNA damage.Our major focus is to understand the cellular responses to lowdoses of radiation that mimic human exposure during diagnosticradiography or occupational activities. We previously observedimportant adaptive responses when normal human cells wereexposed to low doses of137Cs rays. Specifically, these irradiatedcells exhibited significantly less chromosomal damage than oc-curred spontaneously in the corresponding nonirradiated cells(1). To gain insight into this protective mechanism elicited by lowdoses of rays, we used a proteomic approach to identify dif-ferentially expressed proteins. Among many regulated proteins,the translationally controlled tumor protein (TCTP) was found

to be most sensitive, having its level increased by twofold in low-doseirradiated cells.TCTP is highly conserved and abundantly expressed in eukar-

yotes. It is unique in that it has no sequence similarity to otherknown proteins. Homozygous mutant (TCTP/) mice are em-bryonically lethal (2). TCTP is known to participate in numerouscellular processes including protein synthesis and cell growth,thereby explaining the essential nature of the protein in cell sur-

vival. Interestingly, down-regulation of TCTP leads to tumor re-version (reviewed in refs. 3 and 4). In general, TCTP is thought topromote cell viability through an antiapoptotic role (5). However,a role for TCTP in other prosurvival mechanisms is unclear.

Here we tested our hypothesis that TCTP supports the survivaland genomic integrity of irradiated cells through a role in theDNA damage response (DDR). Our data show that TCTP

interacts with ataxia-telangiectasia mutated (ATM) and p53proteins, two major early mediators of DNA damage sensing. Wealso find that in irradiated cells the level of TCTP is regulated inan ATM- and DNA-dependent protein kinase (DNA-PK)de-pendent manner. After DNA damage, TCTP accumulates in thenucleus where it colocalizes at sites of DNA damage with proteinsinvolved in DNA damage sensing and repair. In G1-phase cells, itpromotes binding to damaged DNA of the Ku proteins that arecomponents of the nonhomologous end-joining (NHEJ) mode ofDNA double-strand break (DSB) repair. In TCTP knockdown

cells, DNA damage accumulates, and repair of radiation-inducedchromosomal damage is compromised. TCTP also was found toregulate the radiation-induced G1 and G2 checkpoints, thereforelikely maintaining genomic stability. These data identify a role forTCTP in DNA damage-sensing and repair pathways.

Results

TCTP Induced by Low-Dose Radiation Protects Against DNA Damage.

Micronuclei (MN) are indicators of chromosomal damage andarise mainly from DSBs. When confluent AG1522 normal hu-man fibroblasts were subjected to protracted exposure to low-dose 137Cs rays (10 cGy delivered over 50 h), MN formationdecreased (P< 0.05) even below the spontaneous level (Fig. 1A).Likewise, phosphorylation of p53 on serine 15, another markerof DNA damage, also was reduced (Fig. 1B). These resultssuggest that low-dose radiation triggers mechanisms that protectDNA from basal oxidative damage. To understand the mediatingevents, we used the quantitative amine-specific isobaric tags forrelative and absolute quantitation (iTRAQ) shotgun proteomicsapproach (6). Proteins were isolated from cells immediately afterirradiation and analyzed. Levels of several proteins were foundto be altered relative to sham-irradiated samples (Table S1).

Among the proteins with increased levels, we focused on TCTPand confirmed, by Western blotting, that it was highly up-regu-lated in cells exposed to 10-cGy radiation delivered at a low doserate (0.2 cGy/h) (Fig. 1B). TCTP up-regulation also was inducedafter acute (6 cGy/min) low-dose irradiation and resulted inapproximately sixfold enrichment in nuclei by 4 h after exposure(Fig. 1C). Increased TCTP levels were detected as early as 30

min and as late as 48 h after acute 10 cGy irradiation (Fig. S1A).The induction of TCTP by low-dose rays appeared to bea general phenomenon that occurred in multiple cell strainsgrown in 2D or 3D architecture, in cells at the different phases of

Author contributions: J.Z., S.M.d.T., B.N.P., G.G., D.P., H.L., and E.I.A. designed research;J.Z., S.M.d.T., B.N.P., D.P., and H.L. performed research; J.Z., S.M.d.T., and E.I.A. analyzeddata; and J.Z., D.P., and E.I.A. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail: [email protected].

This article contains supporting information online atwww.pnas.org/lookup/suppl/doi:10.1073/pnas.1106300109/-/DCSupplemental.

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the cell cycle, and in tissues of irradiated mice (Fig. S1 AE).TCTP also was induced following exposure to energetic protons(Fig. S1F).

Incubation of control and -irradiated AG1522 cells for up to30 h with cycloheximide, an inhibitor of cytosolic protein syn-thesis, had no significant effect on TCTP levels (Fig. S1G), in-dicating a long half-life of the protein in these cells. In contrast,p53 was undetectable in the same cells following 4.5 h-incubation(Fig. 1D). Acute irradiation of 10 cGy of rays up-regulatedTCTP by threefold at 4 h after irradiation; incubation in cyclo-

heximide for 0.5 h before irradiation and 4 h postirradiation in-creased TCTP by about the same amount. These results thereforeimply stabilization of TCTP in low-doseirradiated cells.

To determine the protective role of TCTP against DNAdamage, we knocked down the protein in AG1522 cells usingRNAi technology (Fig. S2A). We then assessed the accumulationof DSBs by MN (Fig. 2A) and the formation of H2A.X foci asa function of time after irradiation (Fig. 2B). Confluent cells

were subjected to no or 50-cGy acute-dose irradiation; they weresubcultured 0.25 or 4 h later and were grown for additional 72 hto evaluate MN formation. In irradiated cells transfected withscrambled (Scr) siRNA, the sevenfold increase in MN frequency

observed in cells held in quiescence for 0.25 h decreased by 3.5-fold (P< 0.0001) when the incubation period was prolonged to4 h (Fig. 2A). These expected results are consistent with repair ofDNA damage during the longer incubation period. In contrast,irradiated cells that were treated with siRNA targeting TCTP(siTCTP) showed no decrease in MN formation when held in theconfluent state for 4 h before subculture (P< 0.0001) (Fig. 2A),perhaps because of the failure of DNA repair in the absence ofTCTP. This notion is supported further by the observation that,even in nonirradiated cells, the knockdown of TCTP alsoresulted in an enhanced basal MN level (P< 0.04). These datasuggest that knockdown of TCTP interferes with repair of DNAdamage. This concept was substantiated when similar results

were obtained with Scr siRNA-treated cells that were irradiated

in the presence of the DNA repair inhibitors PJ34 or NU7441,which inhibit poly(ADP-ribose) polymerase and DNA-PK, re-spectively (79). Incubation of the drug-treated and irradiatedcells for 4 h did not result in reduced MN formation (P< 0.0001)(Fig. 2A). Likewise, when Scr siRNA-transfected cells were ex-posed to acute-dose 50-cGy radiation and were held in conflu-ence for 3 or 6 h after irradiation, the number ofH2A.X foci (amarker of DSBs) per cell decreased to 2.6 0.4 and 0.7 0.3,respectively, reflecting ongoing DNA repair (10). By contrast,the foci numbers in TCTP knockdown cells were higher (9.6 1.0,P< 0.001, and 8.6 0.4,P< 0.001, respectively) (Fig. 2B andFig. S2B), suggestive of persistent DNA damage.

Consistent with a function of TCTP in DDR (Fig. 2 A and Band Fig. S2B), Western blot analyses revealed four- to 11-foldincreases in TCTP level in nuclei of AG1522 cells by 0.5 h after

low- (10 cGy), moderate- (50 cGy), or high-dose (400 cGy) acute ray exposure (Fig. 2C), and these observations were confirmedfurther by in situ immunodetection (Fig. 2D, Left). When nu-

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cleoplasmic proteins were removed by detergent treatment, nu-clear TCTP foci were clearly visible, and their number increasedas a function of radiation dose (Fig. 2D, Right). Further, theinduction of TCTP occurred in both the soluble nuclear fractionand the chromatin-enriched fraction of irradiated cells (Fig.S2C). Together, these results (Figs. 1 and 2) strongly supporta role for TCTP in repair and/or sensing of DNA damage in-duced by low to moderate doses of radiation. The protectiveeffects of TCTP against the lethal effects of high therapeutic

doses of rays also were apparent. Knockdown of TCTP en-hanced reproductive failure in AG1522 cells exposed to 200- or400-cGy radiation (Fig. 2E). Although the effect was modest, it

was statistically significant (P< 0.03). These protective effects atboth low and high radiation doses are consistent with prosurvivalfunctions of TCTP (3, 4).

Upstream Regulatory Events. The role of ATM in mediating thecellular responses to DNA damage is well established (11). Toinvestigate whether ATM mediates up-regulation of TCTP bylow-dose rays, we exposed confluent radiosensitive ATMmu-tant cells (AT5B1 and AG4405) to acute-dose 5-cGy radiation.Unlike WT cells (Figs. 1D and Fig. S1 AD), a slight decrease inTCTP level was detected in the mutant cells at 1, 4, or 24 h afterirradiation; representative data at 4 h are shown in Fig. 3A. Themutant cells may suffer from various modifications that mightalter TCTP levels nonspecifically. To rule out this possibility, wetreated moderately radiosensitive WT AG1522 confluent cells

with Ku55933 (10 M), a selective inhibitor of ATM kinase, 0.5 hbefore exposure to acute 10-cGy radiation. Cells were harvestedat 0.5 and 4 h after exposure, and nuclei were extracted. TheTCTP level was found to be decreased (Fig. 3B), suggestinga prominent role of ATM in TCTP expression. By contrast,

vehicle-treated and irradiated WT cells exhibited increased lev-els of TCTP. Importantly, knockdown of ATM by siRNA abro-gated the radiation-induced increase in TCTP level (Fig. 3E).These results show that ATM has a role in regulating and/ormaintaining the increased level of TCTP in cells exposed to low-dose radiation.

ATM belongs to the PI3KK family of proteins. To investigatewhether other members of this family are involved in regulatingthe TCTP level in irradiated cells, DNA-PK WT and mutatedcells were exposed to 10- or 50-cGy radiation. At 0.5 and 3 hpostirradiation, the TCTP level was increased in nuclei of WTbut not mutated cells (Fig. 3C). The role of DNA-PK in regu-lation of TCTP expression in irradiated cells was supportedfurther in experiments with AG1522 cells incubated withNU7441 (Fig. 3D) or treated with siRNA against the catalyticsubunit of DNA-PK, DNA-PKcs, (Fig. 3E). In contrast, knock-down of ATR seemed to have less effect on TCTP up-regulationin irradiated cells (Fig. 3E). Together, the data reveal TCTP asa target of ATM/DNA-PK signaling that is pivotal in the cellularresponse to DNA damage and general oxidative or reductivestresses. It is noteworthy, that inhibition/down-regulation of

ATM, DNA-PKcs, or ATR in nonirradiated AG1522 cellsresulted in increased levels of TCTP (Fig. 3 B and E), suggestingthese proteins have differential effect(s) on TCTP expression incontrol and irradiated cells.

TCTP Exists in Complex with Components of DNA Damage Sensing

and Repair. TCTP also was up-regulated in nuclei of low-dose-irradiated osteosarcoma U2OS cells with abundant ATM (Fig.S2D). We therefore determined if TCTP in these cells interacts

with ATM and/or other proteins involved in DNA damagesensing and repair. A combination of immunoprecipitation (IP),quadrupole TOF (Q-TOF) MS, and immune-blotting showedthat ATM exists in complex with TCTP in nuclei of both controland irradiated cells. Notably, coimmunoprecipitation in chro-matin-enriched fractions revealed a reproducible interaction

between TCTP and ATM in irradiated cells, and these proteinswere found in a complex with H2A.X (Fig. 4A). Further, TCTPand p53, another critical protein involved in DDR (12), also

coimmunoprecipitated reciprocally in nuclear extracts of U2OScells (Fig. 4B). The association of TCTP with p53 was enhancedat 0.5 h after exposure to 50 cGy radiation, suggesting thatradiation-induced lesions may have a role in this interaction atearly stage (Fig. 4B).

Like H2A.X foci, P-ATM (S1981) and 53BP1 foci aremarkers of DSBs. Interestingly, TCTP foci colocalized with all ofthem in AG1522 (Fig. 4 D andE) and U2OS cells exposed to 100cGy-acute dose of radiation (Fig. S3A), thereby identifyingTCTP at sites of DNA breaks. In addition, Q-TOF MS and IP/Western blots identified Ku70 and Ku80, the DNA-bindingsubunits of DNA-PK that participate in NHEJ of DSBs (8, 13), asfactors that form a complex with TCTP in nuclear extracts ofU2OS (Fig. 4C and Fig. S2E) and AG1522 (Fig. S2F) cells.Taken together, these results strongly support a function of

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Fig. 3. Effects of ATM, DNA-PK, or ATR signaling in the regulation of TCTPlevels in -irradiated confluent human cells. (A) Immunoblotting of ATM-mutated cells shows lack of TCTP induction by low-dose rays. (B) Immu-noblots of nuclear extracts of AG1522 fibroblasts exposed to no or 10-cGyradiation in the absence (C) or presence of Ku55933 (10 M), a selectiveinhibitor of ATM. (C) Immunoblot analysis in nuclear extracts from cells thatare WT or mutated for DNA-PKcs. (D) Immunoblots of nuclear extracts fromAG1522 cells irradiated in the presence of NU7441, a selective inhibitor ofDNA-PK. (E) Immunoblots of nuclear extracts from nonirradiated and irra-diated AG1522 cells transfected with Scr siRNA or siRNAs targeting ATM,ATR, or DNA-PKcs.

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TCTP in sensing and repair of DNA damage induced by low(10 cGy), moderate (50 cGy), and high doses (100 cGy) ofionizing radiation.

Knockdown of TCTP Interferes with Subcellular Distribution and DNA-

Binding Activity of Ku70/80 After Irradiation. Compared with cellstransfected with Scr siRNA, depletion of TCTP by siRNA resultedin opposite abundance patterns of Ku proteins in cytoplasm andnucleus of-irradiated cells (Fig. 5A and Fig. S3B). Whereas Ku70and Ku80 proteins were increased in nuclei of control cells by 0.5h after irradiation, presumably because of translocation from thecytoplasm to the nucleus in response to DNA damage (14), thisincrease did not occur in siTCTP-transfected cells. In the latter,Ku70 and Ku80 accumulated in cytoplasm (Fig. 5A, Left and Fig.S3B); in nuclei, the dramatic reduction in TCTP level was asso-ciated with radiation dose-dependent decrease in Ku70 and Ku80(Fig. 5A, Right and Fig. S3B). Relative to Scr siRNA-treated cells,

the decreases in Ku70 and Ku80 abundance in nuclei of irradiatedsiTCTP-transfected cells were associated with significant attenu-ation (>50%, P< 0.001) in the DNA-binding activity of Ku70 andKu80 from extracts of irradiated cells (Fig. 5B). These data furthersupport a role of TCTP in the function of main elements of NHEJmode of DNA repair.

TCTP and Regulation of Radiation-Induced G1 and G2 Delays. Al-though TCTP knockdown inhibits cellgrowthand induces apoptosisin tumor cells (15), down-regulation of TCTP for up to 72 h did notcause apparent structural changes in control AG1522 normal cells(Fig. S2A) or alter their cloning efficiency (28 3%in Scr siRNA vs.29 1% in siTCTP-transfected cells). To examine whether TCTPaffects other cell-growth parameters, we investigated its effect onthe G1- to S-phase transition in control and-irradiated cells. Scr or

TCTP siRNA-treated AG1522 cells synchronized in G0/G1 bydensity-inhibition of growth were exposed to no or 400-cGy radia-tion. Within 10 min after irradiation, theywere subcultured to lower

density and were incubated in the presence of [3H]-thymidine.Movement into the S phase was monitored by autoradiography bymeasuring the cumulativelabeling indices at multiple time points upto 70 h after subculture. The transition time from G1 to S phase innonirradiated cells was similar whether the cells were transfected

with Scr siRNA or siTCTP (Fig. 6A). However, compared with ScrsiRNA-treated cells, a 5.5-h reduction in the radiation-induced G1delay occurred in cells treated with TCTP siRNA (9 h in TCTPsiRNA-treated vs. 14.5 h in Scr siRNA-treated cells) (Fig. 6A). As

well, relative to control, depletion of TCTP attenuated the in-duction of p21Waf1, a cyclin-dependent kinase inhibitor (16, 17) inirradiated confluent cells that were subcultured and harvested foranalyses 24 h later (Fig. 6B, Left). The induction level of p21Waf1

also was reduced at 3 and 24 h after exposure of quiescent siTCTP-

transfected cells to 400-cGy radiation (Fig. 6B, Right). The loss ofnormal G1 checkpoint control disrupts DNA repair and is an earlystep in carcinogenesis (18); therefore, these results are consistent

with a role of TCTP in regulating a process that maintains genomicintegrity.

In addition to its effect on the stress-induced G1 checkpoint,flow cytometry analyses have shown that TCTP has a role inmodulating the magnitude of the radiation-induced G2 check-point in asynchronous AG1522 cells. Similar to the absence of aneffect on G1- to S-phase progression in control cells, knockdownof TCTP did not affect progression through G2 appreciably.However, siTCTP-transfected AG1522 cells exposed to 400-cGyradiation entered G2 phase faster than Scr siRNAtransfectedcells and were arrested in G2 for a longer time (Fig. 6C).Whereas the earlier entry in G2 may be a consequence of the

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Fig. 4. TCTP interacts with components of DNA damage sensing and repair. (A) Benzonase-treated chromatin-enriched fractions that were isolated fromconfluent U2OS cells 30 min after no () or acute [50 cGy, (+)] irradiation (IR) were immunoprecipitated with anti-ATM or anti-TCTP antibodies or normalmouse or rabbit serum (PI). Immunoblots then were reacted with antibodies against ATM, TCTP, H2A.X, or H2A. X. (B) Benzonase-treated nuclear extractsisolated 30 min after exposure of U2OS confluent cells to no or acute (50 cGy) irradiation were immunoprecipitated with anti-TCTP, anti-p53, or control anti-TBP antibodies. Mouse or rabbit preimmune serum (PI) was used as a control. Immunoblotting was performed using antibodies against p53, TCTP, or TBP. (C)Immunoblotting of TCTP, Ku70, and Ku80 in Benzonase-treated nuclear extracts of control nonirradiated U2OS confluent cells after IP with either normalserum (PI) or antibodies against TCTP, Ku70, or Ku80. (D) Untreated or -irradiated (acute, 100 cGy) AG1522 asynchronous cells were preextracted, fixed 1 hlater, and immunostained in situ with anti-TCTP, antiP-ATM (S1981), anti-H2A.X, or anti-53BP1 antibodies. (Scale bars, 10 m.) (E) Quantitative assessmentof colocalization of TCTP foci with those of P-ATM (S1981) (Left), H2A.X (Center), and 53BP1 (Right) in AG1522 asynchronous cells at 1 h after exposure to 50,100, or 200 cGy.

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faster progression through G1 to S phase (Fig. 6A), the longerdelay in G2 likely is caused by a greater level of DNA damage inirradiated cells (Fig. 2).

Discussion

TCTP is thought to protect cells against harmful conditionsthrough antiapoptotic and antioxidant roles (19). The resultsdescribed here identify TCTP as a member of the DDR group ofproteins. Specifically, the protein is essential for eliciting effectiveresponses to radiation-induced genotoxic stress. Our data estab-lish a functional relation between TCTP and several key proteinssuch as ATM, Ku70, Ku80, and p53 that participate in DSBsensing and repair and/or regulation of the radiation-induced cell-cycle checkpoints. They are consistent with recent findings iden-tifying p53 as a TCTP-interacting protein (20). In human cellsexposed to low/moderate doses of rays, TCTP is up-regulatedrapidly in an ATM- and DNA-PKdependent manner (Fig. 3),followed by posttranslational stabilization (Fig. 1D). TCTP formsfoci (Fig. 2D) that colocalize with those of P-ATM (S1981),H2A.X, and 53BP1 that reflect sites of DNA damage (Fig. 4 Dand E). This phenomenon is consistent with the observation thatTCTP interaction with ATM and H2A.X is enhanced in chro-matin-enriched fractions following exposure to ionizing radiation

(Fig. 4A). However, the exact nature of the interaction (direct orindirect) between TCTP and ATM kinase remains unknown.TCTP contains putative PI3KK phosphorylation sites (e.g., T39,S46, and S53), but in experiments involving IP of TCTP from

AG1522 (WT ATM), U2OS (WT ATM), or AG4405 (mutatedATM) cells exposed to 50 cGy of rays, followed by in-gel trypsindigestion and analysis by liquid chromatography tandem MS (LC-MS/MS) on theLTC Orbitrap Velos instrument, we did notdetectany phosphorylation site within the protein.

Further, TCTP coimmunoprecipitated with the Ku proteins (Fig.4C) involved in NHEJ of DSBs. TCTP therefore may participate inrepair of DSBs. This notion is substantiated further by the obser-

vation that TCTP knockdown prevents the repair of radiation-induced DNA damage leading to the accumulation of MN andH2A.X foci (Fig. 2 A and B and Fig. S2B). The DSB is a serious

DNA lesion that leads to cell death (21). Consistent with thisconcept, the accumulation of DNA damage in siTCTP cellsresults in enhanced cell killing (Fig. 2E). Importantly, TCTP isrequired for the DNA-binding activity of Ku70 and Ku80 in re-sponse to irradiation (Fig. 5B). Moreover, in TCTP-depleted cellsexposed to radiation, Ku70 and Ku80 levels in nuclei are reduced(Fig. 5A and Fig. S3B), probably by disruption of the events im-plicated in Ku translocation to the nucleus. This reduction mayimply a chaperone role of TCTP as described previously (22). Inthis context, our IP and MS analyses indicated that in irradiatedcells TCTP also interacts with the stress-induced molecularchaperones, heat shock 90-kDa, 70-kDa, and 60-kDa proteins.

Damage to DNA in the form of DSBs can lead to carcinogenesisand cell death (23). Two mechanisms participate in the repair ofDSBs: homologous recombination repair (HRR), acting mainly inthe S and G2 phases of the cell cycle, and NHEJ, acting throughoutthe cell cycle (13). Our data show that TCTP has important effectson the Ku proteins. This finding is particularly important becauseKu proteins are involved in DSB repair by NHEJ, and inactivationof Ku proteins leads to defects in telomere maintenance andchromosomal end fusion (24). Our IP/MS experiments also in-dicate that TCTP interacts with filamin A, which appears to berequired for efficient HRR (25). The role of TCTP in such criticalmechanisms that maintain genomic integrity may explain in part

why homozygous mutation in TCTP is embryonically lethal (2).TCTP is coimmunoprecipitated with p53 (Fig. 4B), a protein

with an essential function in the radiation-induced G1 checkpoint

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Fig. 5. TCTP is involved in regulation of subcellular distribution and DNA-binding activity of Ku70 and Ku80 after irradiation. (A) Confluent U2OScells transfected with Scr or TCTP siRNA were exposed to no, 50-, or 370-cGyradiation. Expression of TCTP, Ku70, and Ku80 in nuclei and cytoplasm

fractions 30 min after irradiation was examined by Western blotting. Re-action of goat anti-rabbit IgG (sc-2030; Santa Cruz Biotechnology) witha protein of 30 kDa was used to verify equal loading of samples (loadingcontrol). (B) Ku70 (Right) and Ku80 (Left) DNA-binding activity in irradiatedU2OS confluent cells expressing Scr or TCTP siRNA at 30 min after irradiation.

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Fig. 6. Effect of TCTP knockdown in the -rayinduced G1 and G2 check-points in AG1522 fibroblasts. (A) Confluent cultures of Scr or TCTP siRNA-treated cells were exposed to no or 400-cGy radiation, were subcultured tolower density within 10 min after irradiation, and were grown in the pres-ence of [3H]-thymidine. Cumulative labeling indices were measured as afunction of time after subculture. (B) Western blot analyses reveal thatknockdown of TCTP attenuates p21Waf1 induction in irradiated confluentcells that subsequently were subcultured to lower density or were main-tained in the confluent, density-inhibited state. (C) The -rayinduced G2

delay in Scr or siTCTP asynchronous cells.


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(12). The knockdown of TCTP attenuates the -rayinduced G1delay in p53 WT AG1522 cells but has no effect on normal pro-gression from G1 to S phase (Fig. 6A). The attenuated stress-in-duced G1 checkpoint in these cells correlates well with theattenuated increase in the levels of p53 and of p21Waf1 thatinhibits the activity of cyclin-dependent kinases involved in pro-liferation (Fig. 6B). A reduction in the duration of the G1checkpoint presumably deprives irradiated cells of crucial time torepair DNA damage before DNA replication. These data are

consistent with yet another role for TCTP in p53-dependentmechanisms that maintain genome integrity in normal cells understressful conditions. Further, knockdown of TCTP modulates the-rayinduced G2 checkpoint (Fig. 6C). Ten to twenty-five hoursafter exposure to 400 cGy, greater fractions of cells were delayedin G2 phase, likely as a result of accumulation of DNA damage inthese cells (Fig. 2). Recently, overexpression of TCTP was shownto destabilize p53 in lung carcinoma cells (20), and knockdown ofTCTP interfered with the p53MDM2 axis (26). Therefore TCTPeffects on p53 are multifaceted and may depend on cell type,cellular microenvironment, inherent sensitivity, and whether cellsare challenged with low or high doses of environmental agents.

Low doses of toxic agents, including chemicals and ionizing ra-diation, often induce protective mechanisms that enhance the ability

of the organism to cope with stress from normal metabolism orexposures to exogenous agents (27). Here, we show that doses ofrays as low as 110cGy (Fig. S1AE), particularly when delivered ata low dose rate (0.2 cGy/h), up-regulate TCTP expression and re-duce the frequency of MN formation to below the spontaneousrate in human fibroblasts (Fig. 1A). MN arise predominantly fromDSBs, andin the case of spontaneous formation, such as when TCTP

was knocked down in AG1522 cells (Fig. 2A), they likely are inducedby closely spaced oxidative base lesions caused by reactive oxygenspecies generated during normal cellular metabolism (23). TheseMN formation results and those showing that knockdown of TCTPattenuates the repair of radiation-induced DNA breaks (DSBs,closely spaced single-strand breaks) that lead to MN formation andH2A.X foci (Fig. 2) suggest a role for TCTP in the expression ofradiation-induced adaptive responses (27, 28).

TCTP up-regulation by ionizing radiation is not restricted to tis-sue-culture phenomena but also occurs in tissues of mice after low-dose irradiation to the whole body (Fig. S1E). The fact that TCTPinduction occurs in cells at all phases of the cell cycle and is of long(at least 48 h) duration (Fig. S1A) is consistent with its role asa surveillance sentinel that guards against prolonged radiation-in-duced oxidative damage (29). In addition, TCTP also is responsiveto high-dose irradiation (Figs.2, 5, and 6).Understanding the role ofTCTP in the sensing and repair of radiation-induced DNA damageas described here (schematic in Fig. 7) will be informative for un-derstanding the system responses to low-dose radiation exposures,and this understanding would help in estimating the health risks ofsuch exposures. It also may lead to greater insight in the molecularevents induced by therapeutic doses of radiation.

Materials and MethodsCells. The human skin fibroblast strains AG1522 and GM5758 are apparentlynormal; AT5BI and AG4405 were derived from patients with ataxia telan-giectasia (Coriell Institute for Medical Research, Camden, NJ). U2OS cells areosteosarcoma cells (ATCC). M059K (WT) and M059J (DNA-PKmutated cells)are human glioblastoma cells (ATCC). Unless indicated, confluent cells wereused in experiments and were grown as described (1). For growth in 3Darchitecture, cells were adapted to grow on a Cytomatrix carbon scaffold(Cell Science Therapeutics) (1). Experiments were repeated two to six times,and comparisons between treatment groups and respective controls wereperformed using the Pearsons 2-test. A P value of 0.05 between groupswas considered significant.

Animals. The 5- to 6-wk-old C3H/HeJ mice were obtained from JacksonLaboratory. When irradiated, they were 7 to 9 wk old.

Irradiation. Cell cultures were exposed to rays at 37 C in a humidified at-mosphere of 5% (vol/vol) CO2 inair in aMark I

137Cs irradiator (J. L. Shepherd)at a lowdoserate (0.2 cGy/h or 6 cGy/min) or an acute dose rate (330 cGy/min).Exposure to 1 GeV protons was carried at the National Aeronautics SpaceAgency Space Radiation Laboratory (Upton, NY) at 5 cGy/min.

Inhibitors. PJ34 (Alexis Biochemicals) was used at 30 M and was added tocells 3 h before irradiation. Ku 55933 (KuDOS Pharmaceuticals) and NU7441(Tocris) were added at 10 M 30 min before irradiation. Cycloheximide(Calbiochem) was added at 2 g/mL 30 min before irradiation. Cells wereincubated with the various inhibitors until harvest. Controls were incubatedwith the dissolving vehicles.

Immunoblotting and Antibodies. Immunoblotting was performed as described(1). The primary antibodies were TCTP [ab37506 (Abcam) and sc-30124(Santa Cruz Biotechnology)]; ATM [sc-23291 (Santa Cruz Biotechnology),A1106 (Sigma), and GTX 70103 (GeneTex)]; P-ATM (S1981) (05-740; UpstateBiotechnology); ATR (A300-138A; Bethyl Laboratories); DNA-PKcs (sc-5282;Santa Cruz Biotechnology); Ku70 (sc-1486 and sc-9033; Santa Cruz Bio-technology); Ku80 [sc-9034 (Santa Cruz Biotechnology) and GTX 22173(GeneTex)], p53 [OP03 and OP43 (VWR) and 9282 (Cell Signaling)]; P-p53(S15) (9284; Cell Signaling); p21Waf1 [OP64, Ab-1 (VWR), 05 345MI (Fisher),and 53BP1 (A300-272A; Bethyl Laboratories)]; H2A.X (05-636MI; Fisher);H2A.X (50-230-9763; Fisher); and ORC2 (559255; BD Biosciences). Secondaryantibodies for Western blotting were from BioRad. To verify equal loadingof samples, membranes were stained with Ponceau S Red (Sigma) andreacted with anti-tubulin (CP06; Calbiochem), anti-TATA box-binding pro-tein (TBP) (ab818; Abcam), or goat anti-rabbit IgG (sc-2030; Santa Cruz

Biotechnology) that recognizes a protein of 30 kDa (loading control).

Immunofluorescence. The cells were fixed in 4% (wt/vol) formaldehyde for 10minat room temperature, permeabilizedin 0.2%TritonX-100in PBSfor 10 min,blocked with 4% (wt/vol) BSA, and incubated with the primary antibodies.Signals were visualized by use of secondary antibodies conjugated with AlexaFluor 488, Alexa Fluor 594, or DAPI (Invitrogen). For preextraction, cells weresubjected to detergent extraction with Triton X-100 in PBS (0.5% for 510 min)to remove the majority of nonchromatin-bound proteins before fixation andimmunostaining.

Foci Analyses. Digital images were acquired from random fields (Zeiss Axi-overt 200M) and analyzed with AxioVision LE 4.6 software. The Apotomefunction of the microscope was used for foci colocalization. Freely availableFociCounter software (30) was used to count foci, and colocalization wasscored manually in at least 30 randomly chosen cells.

IR

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Fig. 7. The role of TCTP in DNA damage sensing and repair. TCTP is up-regulated by ionizing radiation, interacts with elements of DNA damagesensing and repair, and modulates radiation-induced cell-cycle checkpoints.HR, homologous recombination; HSPs, heat shock proteins; IR, ionizing ra-diation; MRN, MRE11RAD50NBS1; NHEJ, nonhomologous end joining; P,

phosphorylation.

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IP. Monolayer cells were washed with PBS and lysed onicefor10 min inbuffer A[10mM Hepes (pH 7.9),1.5 mMMgCl2, 10 mMKCl,0.5 mM DTT,0.05% NonidetP-40 with protease and phosphatase inhibitor mixtures] (Sigma). After cen-trifugation (800 g for 10 min at 4 C), the nuclei pellet was resuspended inhigh-salt buffer B [5 mM Hepes (pH 7.9), 300 mM NaCl, 1.5 mM MgCl 2, 0.2 mMEDTA, 0.5 mM DTT, 26% glycerol (vol/vol), and 0.5% Nonidet P-40] and left onice for 30 min. Supernatants were collected by centrifugation at 14,000 g for20 min at 4 C and were treated with 100 U/mL Benzonase (EMD Millipore) for1 h at room temperature or with 50 g/mL ethidium bromide on ice for 30 minto release chromatin-bound proteins. Following incubation with appropriate

antibodies overnight at 4 C, a mixture of protein A and protein G beads(Pierce) was added,and the mixture was incubated for>3 h at4 C. After threewashings with Hepes-buffered saline buffer [50 mM Hepes (pH 7.5), 1 mMEDTA, and 0.2% Nonidet P-40], proteins were solubilized in SDS/PAGE loadingbuffer and analyzed by Western blotting. Inputs represent 1/20 of the extractused for IP.

Subcellular Fractionation. For nuclei and chromatin-bound protein isolation,107 cells were lysed in 1 mL of buffer A [10 mM Hepes (pH 7.9), 1.5 mMMgCl2, 10 mM KCl, 340 mM sucrose, 10% (vol/vol) glycerol, 0.1% Triton X-100, 1 mM DTT, 0.1 mM PMSF, protease inhibitor mixture] for 10 min on ice.Cytoplasmic proteins were separated from nuclei by low-speed centrifuga-tion (500 g for 5 min at 4 C). Isolated nuclei were washed once with bufferA and then lysed in buffer B (3 mM EDTA, 0.2 mM EGTA, 1 mM DTT, andprotease inhibitors) on ice for 15 min. Soluble nuclear proteins were sepa-rated from insoluble chromatin by centrifugation at 1,500 g for 5 min at 4

C. Isolated chromatin was washed with buffer B and centrifuged at 10,000 gfor 1 min. The final chromatin pellet was resuspended in buffer A withoutglycerol. Then 100 U/mL Benzonase (EMD Millipore) was added and incubatedfor 1 h at room temperature to release chromatin-bound proteins.

Ku70 and Ku80 Activity. Ku70 and Ku80 DNA-binding activity was analyzed bya quantitative Ku70/80 DNA repair kit (Active Motif). Briefly, cells werewashed and resuspended in hypotonic buffer. The nuclear extract was iso-lated forKu activity analysis.Then 5 g of nuclear protein wasincubated withimmobilized, blunt-ended linear DNA in wells of a 96-well plate for 1 h atroom temperature. Bound Ku70 or Ku80 was detected using respectiveantibodies followed by detection with HRP-linked secondary antibody. Eachsample was assayed with Ku competitor oligonucleotide to ensure specific-ity, and a positive control consisting of a nuclear lysate of Raji cells, whichare provided with the kit, was included in every run. The color product wasdetected at 450 nm with use of a Perkin-Elmer Victor plate reader, andabsorbance values were corrected for assay blank (activity in the absence ofsample) and nonspecific signal (activity in the presence of competitor). Eachexperiment was repeated three times, and data represent the mean SD ofthree separate determinations.

siRNA Silencing. siRNAs capable of targeting mRNAs encoding TCTP (alsoknown as tpt1) were from Ambion. Scr siRNA Duplex (Ambion) was includedas control. The siRNAs to knock down ATM, ATR, and DNA-PKcs were fromThermo Scientific. Treatments were performed 72 h posttransfection whenthe cells were in G0/G1 phase of the cell cycle.

MN Formation. DNA damage and its repair were assessed by measuring thefrequency of MN formation by the cytokinesis-block technique as previouslydescribed (1). After treatments, confluent AG1522 fibroblasts were sub-cultured, and 3 104 cells were seeded in chamber flaskettes (Nunc) andallowed to progress in the cell cycle in the presence of 2 g/mL cytochalasin B(Sigma). After 72 h incubation, an ample time to account for radiation-inducedcell-cycle delays and for replication of AG1522 cells that have a doubling timeof 26 h, the cells were rinsed in PBS, fixed in cold methanol, stained withHoechst 33342 (1 g/mL PBS), and viewed with a fluorescence microscope. At

least 1,000 cells per treatment were examined, and only MN in binucleatedcells were considered for analysis. At the concentration used, cytochalasin Bwas nontoxic. Binomial statistics were applied in analysis of the data.

Cell Survival. Confluent cultures were exposed to rays and assayed forclonogenic survival within 510 min after exposure.

G1 to S Phase Traverse. Quiescent cell populations were subcultured to lowdensity in growth medium containing [3H]-thymidine (1 Ci/mL, specific ac-tivity 20 Ci/mmol) (PerkinElmer LAS, Inc), seeded in 30-mm dishes, and in-

cubated at 37 C. At regular intervals, duplicate dishes were rinsed with PBS,fixed with ethanol, and subjected to autoradiography. To determine label-ing indices, a minimum of 1,000 cells per dish was scored. The use of thiscontinuous-labeling technique allows precise determination of G1 delays.

Delay in G2 Phase. Actively growing cells were -irradiated, and samples wereharvested by trypsinization at different times after exposure. They wererinsed in PBS, fixed in 4% (wt/vol) formaldehyde for 10 min at room tem-perature, and then permeabilized in 0.2% Triton X-100 in PBS/1% BSA for 10min. Following RNase treatment (0.5 mg/mL in PBS/1% BSA) at 37 C for 1 h,the cells were stained with propidium iodide and submitted to flowcytometry (FACSCalibur; Becton Dickinson).

iTRAQ Labeling, MS, and Protein Quantification. Soluble proteins were se-quentially reduced, alkylated, digested, and labeled with four iTRAQreagents according to the manufacturers instructions (Applied Biosystems).The labeled peptides were combined and separated by strong cation-ex-change chromatography followed by reversed-phase LC and analysis ona 4700 Proteomics Analyzer tandem mass spectrometer (ABI) (6). GPS Ex-plorer software (v. 3.5; ABI) was used to process the MS/MS spectra and tosubmit peak lists to the MASCOT (v. 1.9) search engine for peptide identi-fication. Only peptides identified with confidence interval values 95%were used for protein identification and quantification.

Phosphorylation Sites in TCTP. AG1522, U2OS, or AG4405 cells were exposedto 50 cGy of rays and incubated for 0.5 h. IP with anti-TCTP was carried innuclear extracts. Samples were fractionated in SDS/PAGE, and the TCTP bandwas excised for in-gel trypsin digestion. The resulting peptides were ana-lyzed by LC-MS/MS on an Orbitrap Velos instrument (Thermo Fisher Scientific).The peptides were separated by a reversed-phase column (75 m 150 mm, 3m, 100 , C18; Dionex) on an Ultimate 3000 LC system (Dionex) and were

introduced directly into a Proxeon nano-electrospray ionization source onthe Orbitrap Velos instrument for MS/ MS analysis. The MS spectra wereacquired in the positive ion mode with a spray voltage of 2 kV and a capillarytemperature of 275 C. Collision-induced dissociation was used for peptidesfragmentation. The MS/MS spectra were searched against the UniProt hu-man database using both Mascot and Sequest search engines. The protein-sequence coverages for TCTP were 87% for all three samples.

ACKNOWLEDGMENTS. We thank Dr. R. W. Howell for input in low-dose-rateradiation studies, the staff at the National Aeronautics Space AdministrationSpace Radiation Laboratory for proton irradiation, and Drs. U. Herbig andA. Asaithamby for discussions. E.I.A. was supported by Grant DE-FG02-07ER64344 from the US Department of Energy, Low Dose Radiation Re-search Program; Grant NNJ06HD91G from the National Aeronautics andSpace Administration; and Grants AI067773-04 (a National Institute of Al-lergy and Infections Disease subaward) and CA049062 from the NationalInstitutes of Health. H.L. was supported by National Institutes of Health

Grant NS046593. D.P. was supported by Grant AG030504 from the NationalInstitute on Aging and by Grant 09GRNT2260364 from the American HeartAssociation. J.Z. received support from the Program for Changjiang Scholarsand Innovative Research Team, University of Ministry of Education, China.

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normal human fibroblasts: The role of growth architecture and oxidative metabolism.

Radiat Res 166:849857.2. Chen SH, et al. (2007) A knockout mouse approach reveals that TCTP functions as an

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Supporting Information

Zhang et al. 10.1073/pnas.1106300109

0 10 0 10 cGy

AG1522

TCTP

-tubulin

Ratio:

TCTP/ -tubulin1.0 2.4

GM5758

1.0 2.2

3D cultures

1.0 2.1 3.8 4.7

0 1 5 10 cGy

TCTP

-tubulin

Ratio:

TCTP/ -tubulin

0 100 10 cGy

1h 24h

-tubulin

1.0 2.2

TCTP

1.0 2.4Ratio:TCTP/ -tubulin

Brain Lung

1.0 1.9

0 10

1.0 2.0

0 10 cGy

TCTP

-tubulin

Ratio:

TCTP/ -tubulin

24h 30h

- + - +

p53

TCTP

Ratio

TCTP/TCTP1.0 0.8 1.0 1.1

CH

A B C

D E GF

0 10 cGy

1.0 1.8

TCTP

-tubulin

Ratio:

TCTP/ -tubulin

1h 48h

-tubulin

0 10 0 10 cGy

TCTP

TCTP

TCTP

24h

30h

48h

Time

after

subculture

-tubulin

-tubulin

Fig. S1. Western blot analyses of translationally controlled tumor protein (TCTP) in normal human cells and in tissues of mice exposed to low-dose 137Cs rays

or 1 GeV protons. (A) Up-regulation of TCTP in asynchronous cells. Confluent normal human AG1522 cells were subcultured and exposed to acute rays at

different time points: 24 h later, when the population was enriched in S-phase cells; at 30 h, when the population was enriched in G 2/M cells; and at 48 h, when

dividing cells progressed in the following growth cycle. Cells were harvested 1 or 48 h after irradiation, and TCTP expression was quanti fied. (B) TCTP was

induced up to 24 h after exposure of confluent AG1522 cells to acute 10 cGy of rays. (C) TCTP expression in confluent AG1522 or GM5758 normal human skin

fibroblasts at 4 h after no exposure or acute exposure to 10 cGy of rays. (D) TCTP up-regulation in irradiated AG1522 cells maintained in a 3D architecture.

Cells were exposed to low-dose rays and harvested 4 h later. (E) TCTP is up-regulated in tissues of C3H/HeJ mice exposed to whole-body irradiation by low-

dose rays. Tissues were harvested for analysis 4 h after exposure. (F) TCTP is up-regulated in confluent AG1522 cells exposed to 1 GeV protons and harvested

24 h later. (G) Western blot analyses of TCTP in AG1522 confluent cell cultures exposed to 10 cGy irradiation at an acute dose rate (6 cGy/min) in the presence or

absence of cycloheximide (2 g/mL). Cells were incubated in cycloheximide for 30 min before irradiation and were harvested for analysis 24 or 30 h later.

Zhang et al. www.pnas.org/cgi/content/short/1106300109 1 of 4
http://www.pnas.org/cgi/content/short/1106300109http://www.pnas.org/cgi/content/short/1106300109


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TCTP H2A.X DAPI Merge

TCTP P-ATM-S1981 DAPI Merge

TCTP 53BP1 DAPI Merge

A

BKu70 H2A.X DAPI Merge

Untreated

3

70cGy

Ku70 H2A.X DAPI Merge

Scr siRNA

TCTP siRNA

Untreated

370cGy

Fig. S3. (A) TCTP colocalizes with P-ATM (S1981), H2A.X, and 53BP1 following 137Cs irradiation. U2OS cells exposed to 100-cGy radiation were preextracted

and were fixed 1 h later; they were immunostained with anti-TCTP, anti-P-ATM (S1981), anti-H2A.X, or anti-53BP1 antibodies. (Scale bars, 10 m.) (B) U2OS

cells transfected with Scr or TCTP siRNA were left untreated or were exposed to 370-cGy radiation. The cells were fixed 0.5 h later and were immunostained in

situ with anti-Ku70 (red) and anti-

H2A.X (green) antibodies. (Scale bars, 10

m.)



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Table S1. Top-ranked proteins modulated in AG1522 confluent cells exposed to 10 cGy of 137CS rays (0.2 cGy/h) as revealed by the

isobaric tags for relative and absolute quantitation shotgun proteomics approach

Change UniProt accession no. Protein name Ratio* SD

Increase P13693 Translationally controlled tumor protein (TCTP) (p23) (histamine-releasing factor) (HRF) 1.811 0.072

Increase P04083 Annexin A1 1.618 0.292

Increase Q16740 Putative ATP-dependent Clp protease proteolytic subunit, mitochondrial precursor 1.566 0.293

Increase P11413 Glucose-6-phosphate 1-dehydrogenase 1.544 0.252

Increase P04264 Keratin, type II cytoskeletal 1 (Cytokeratin 1) 1.499 0.495

Increase P17317 Histone H2A.Z (H2A/Z) 1.495 0.16Increase Q9P2J5 Leucyl-tRNA synthetase, cytoplasmic 1.466 0.091

Increase P08253 72-kDa type IV collagenase precursor 1.456 0.223

Increase Q15366 Poly(rC)-binding protein 2 1.445 0.499

Increase Q10567 Adapter-related protein complex 1 beta 1 subunit 1.435 0.114

Increase P51991 Heterogeneous nuclear ribonucleoprotein A3 1.433 0.081

Increase O60664 Mannose-6-phosphate receptor binding protein 1 1.421 0.195

Increase Q13596 Sorting nexin 1 1.407 0.093

Increase O43399 Tumor protein D54 (hD54) 1.4 0.089

Increase P82675 Mitochondrial 28S ribosomal protein S5 (S5mt) 1.395 0.094

Increase Q13151 Heterogeneous nuclear ribonucleoprotein A0 (hnRNP A0) 1.391 0.047

Increase O95340 Bifunctional 3-phosphoadenosine 5-phosphosulfate synthethase 2 (PAPS synthethase 2) 1.362 0.333

Increase P48681 Nestin 1.357 0.318

Increase Q15046 Lysyl-tRNA synthetase 1.35 0.078

Increase P09651 Heterogeneous nuclear ribonucleoprotein A1 1.325 0.028

Increase Q8WWX9 Selenoprotein M precursor 1.319 0.08Increase P23381 Tryptophanyl-tRNA synthetase 1.317 0.329

Decrease P62873 Guanine nucleotide-binding protein G(I)/G(S)/G(T) beta subunit 1 (Transducin beta chain 1) 0.613 0.38

Decrease P99999 Cytochrome c 0.620 0.403

Decrease P10620 Microsomal GST 1 0.714 0.235

Decrease Q15629 Translocation associated membrane protein 1 0.717 0.194

Decrease P35556 Fibrillin 2 precursor 0.732 0.01

Decrease P61619 Protein transport protein Sec61 alpha subunit isoform 1 0.735 0.134

Decrease P30049 ATP synthase delta chain, mitochondrial precursor 0.736 0.099

Decrease P53680 Clathrin coat assembly protein AP17 (Clathrin coat associated protein AP17) 0.746 0.155

Decrease Q9Y3D6 Tetratricopeptide repeat protein 11 0.748 0.286

*Ratio: protein level in irradiated vs. nonirradiated cells.


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Role of the Translationally Controlled Tumor Protein

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